1. Field of the Invention
The present invention relates to small water plane area vessels (also referred to "SWAS" vessels) and more specifically to a SWAS vessel having improved hydrodynamic stability, low water resistance, and minimal ship motion.
2. Background of the Invention
Small water plane area ships (SWAS) generally consist of a vessel having at least one water line located below its design draft with a water plane area that is significantly larger than the water plane area at its design draft. One form of such vessel is a small water plane twin hull vessel (also referred to as a SWATH vessel) which generally consists of two submerged hulls, originally formed of uniform cross section, connected to a work platform or upper hull by elongated struts which have a cross section along any given water plane area that is substantially smaller than a water plane area cross section of the submerged hulls. Thus, at the design water line, with the hulls submerged, such vessels have a small water plane area.
SWAS vessels may have one or more lower hulls connected to the work platform or super structure by one or more struts. Originally, SWAS vessels utilized single struts between two submerged hulls and the upper platform, as shown for example in U.S. Pat. Nos. 3,447,502 issued to Lang, and 4,552,083 issued to Schmidt. Some time ago, however, the Naval Ocean System center at San Diego and Honolulu developed a SWAS design characterized by having a least two struts associated with each submerged hull. These vessels are further characterized by submerged twin hulls with uniform cross sections and at least two narrow struts making a connection, at the forward and aft ends of the submerged hulls and the platform. These struts typically extend vertically, as shown for example in U.S. Pat. Nos. 3,623,444 and 3,897,944, issued to Lang. Other forms of such vessels have been disclosed which contain a single lower hull connected by one or more struts to the work platform and vessels having three or more lower hulls connected to the work platform by one or more struts associated with each hull.
SWAS vessels of this type usually include sponsons (alternatively referred to in the art as upper hulls or upper struts) which are structures positioned above the struts and below the work platform or the super structure that have significantly increasing water plane areas extending from the strut to the platform. That is, these sponsons are flared hull type structures in cross section having deadrises extending along the length of the vessel. The sponsons may be continuous or segmented over each strut. The struts themselves are generally foil shaped and constant in cross sectional areas. However, as is known in the art, these struts can also be tapered and/or can be canted at negative or positive diehedral angles.
In SWAS vessels, it is desirable to maintain a minimum water plane area at the design water line for most efficient operation of the vessel. However, this desirable goal is limited by the need for a minimum water plane area required to maintain hydrostatic stability. As a result, existing SWAS vessels commonly have a problem with trim and heel stability due to the small water plane area of the struts. These vessels also suffer from high frictional drag due to relatively large surface areas formed by the struts despite every effort that has been made to minimize this.
It has been found desirable that the aft sections of the submerged hull in SWAS vessels be streamlined in order to improve propulsion efficiency. However, streamlining the lower hulls (i.e. tapering their aft sections) causes significant losses in buoyancy at a position in a vessel where it is usually essential.
The use in SWAS vessels of sponsons having greatly flared cross sections with low to moderate deadrise has been proposed in order to meet static and damage hydrostatic ability requirements. In other words, should the vessel be compromised and take on water, as the submerged hulls and struts sink in the water, the sponsons engage the water surface to increase buoyancy of the entire vessel. However, the presence of the sponsons, which are located above the water line during normal operation of the vessel, create slamming in high seas and contributes to increased ship motion and resistance in heavy weather. Such slamming can of course cause significant structural damage.
Although SWAS vessels have been found to be highly desirable in a number of applications, the above problems present a continuing need for improved design. Thus, it has been found that there is a need to have a simple and cost effective way to modify the location of the centers of gravity, buoyancy and flotation in such vessels under various operating conditions. It is further desirable to provide adequate water plane area for such ships which generally have multiple design operating drafts.
It is an objective of the present invention to provide a simple and cost effective way to modify the centers of gravity, buoyancy and flotation of SWAS vessels under various operating conditions.
Another object of the present invention is to provide a reduced water plane area for SWAS vessels which have multiple design operating drafts.
Yet another object of the present invention is to reduce ship motions in SWAS vessels in order to advantageously exploit the Froude Krylov forces acting on such vessels.
A further object of the present invention is to reduce SWAS vessel motions through added mass and viscous damping.
Yet a further object of the present invention is to reduce wetted surface area in SWAS vessels while maintaining adequate displacement.
A still further object of the present invention is to provide an improved SWAS vessel.
Another object of the present invention is to provide a SWAS vessel in which the geometric conditions of the vessel call be modified to obtain improved operating conditions.